The present invention is related in general to the field of semiconductor devices and electronic systems and more specifically to reduced-error set-ups and operation of computer-controlled bonding machines used in integrated circuit assembly.
In integrated circuit (IC) assembly, an IC chip is typically mounted on a leadframe and electrically connected to it by metallic segments. Commonly, the chip assembly is encapsulated in a protective package (for instance, ceramic package, or plastic package using molding process). Typically, the IC chip has a plurality of bond pads, which are often positioned around the chip perimeter; these bond pads have predetermined bonding area and spacing (bond pad pitch). The leadframe usually has a plurality of narrow xe2x80x9cinnerxe2x80x9d leads for attachment to the segments and inclusion in the package, and a plurality of wider xe2x80x9couterxe2x80x9d leads for attachment to other parts such as solder attachment to circuit boards.
The metallic segments used for electrical connection of the IC chip to the leadframe include wires and ribbons, and are attached by ball bonding, stitch bonding, or wedge bonding techniques. Wire bonding is a process in which a wire may be welded from a chip bond pad to the tip of an inner lead of the leadframe. As an example, in wire ball bonding the ball is attached to the chip bond pad and the stitch to the leadframe inner lead. For a given device type, there is a set of locations expressed in x and y coordinates which defines the bond locations on the chip and on the lead tips. These locations are generally stored collectively in a computer file, sometimes referred to as xe2x80x9cDevice Programxe2x80x9d. Apart from the bond head, capable of providing x-y-z motion needed for bonding, a wire bonder has a material handling subsystem and the vision subsystem.
Conventional semiconductor computerized wire bonders use x-y tables to move the bonding capillary over the device for bonding between the chip and the leadframe. The x-y coordinate tables are driven by complex electrical and mechanical components that may convert rotary and linear motions of the axis drive motors to create the needed positioning. The bond head also carries several other components such as the z-axis drive motor, a camera and optics for vision functions, and further components required to control wire bonding. Specific features of the capillary and its alignment are described in a number of U.S. patents and patent applications. Examples are: U.S. Pat. No. 5,934,543, issued on Aug. 10, 1999 (Koduri et al., xe2x80x9cWire Bonding Capillary having Alignment Featuresxe2x80x9d); and application Ser. No. 08/993,638, filed on Dec. 18, 1997 (Koduri, xe2x80x9cWire Bonding with Capillary Realignmentxe2x80x9d). The interaction of capillary and vision system is illustrated, for example, by U.S. patent application Ser. No. 09/191,812, filed on Nov. 13, 1998 (Koduri et al., xe2x80x9cAutomation of Optics Offset Measurement on Wire Bondersxe2x80x9d); Ser. No. 09/111,642, filed on Jul. 8, 1998 (Koduri et al., xe2x80x9cAn Efficient Hybrid Illuminatorxe2x80x9d); Ser. No. 09/111,977, filed on Jul. 8, 1998 (Koduri et al., xe2x80x9cAn Efficient Illumination System for Wire Bondersxe2x80x9d).
The material handling system moves a leadframe so that each device can eventually be placed under the bond head for bonding. One or more devices may be placed under the bond head at a time to be bonded. The device may also be heated in a predetermined manner to establish reliable metallurgical bonding conditions. After a device has been bonded, the leadframe is step-moved such that the next unit can be bonded.
When a unit is indexed in by the material handling system for bonding, the position of the leadframe and the chip is not always the same because of variations in the handling and previous manufacturing (such as variable chip positioning during attachment to the leadframe). Without knowing accurately the target bonding locations, the bond head cannot place the bonds as expected. To aid this process, a machine vision system is employed. A typical vision system consists of a set of optics to provide the needed illumination and magnification of the device, a camera to capture the image provided by the optics and an image processing system to store and analyze the captured image.
Before bonding a device, it is essential to determine the device program with all the coordinate locations of the bonds that need to be created. With respect to a predetermined set of reference locations, those locations are often referred to as xe2x80x9chomesxe2x80x9d. A typical device may have one or more xe2x80x9chomesxe2x80x9d. Generally, the identification of homes needs to be done individually for each device to be bonded. It is common practice to use a three-step process to enable such identification.
In the step of xe2x80x9cteachingxe2x80x9d, the coordinate locations of the homes and all the needed bonds are identified and saved to create the xe2x80x9cdevice programxe2x80x9d. Once generated, a device program can be stored, copied and/or shared between multiple machines as needed.
In the step of xe2x80x9cregenerationxe2x80x9d, a human operator helps in locating the homes of the first device after loading in the information from the previously saved device program. At this point, the machine captures and saves a set of images, called xe2x80x9creference imagesxe2x80x9d or xe2x80x9creferencesxe2x80x9d in the neighborhood of each home.
In the step of xe2x80x9cbondingxe2x80x9d, the machine indexes one unit or more at a time into the workstation under the bond head. At this point, the vision system, with the aid of a pattern recognition system, attempts to relocate the matching locations with the saved references. After finding the new coordinates of the matching references, the home and bond locations are re-computed for that specific unit from the device program data. The process of relocating the references and homes is normally referred to as xe2x80x9caligningxe2x80x9d the device. Using the specific bond locations, the device can now be bonded. The process of indexing, aligning and bonding is repeated without any human intervention as long as nothing abnormal happens on the machine.
A typical alignment procedure may correct for a constant shift in x-y directions and/or a constant rotation of the device. In this context, it is important to understand the effects of variations in illumination settings across machines and the images formed using different levels of brightness. Large differences in intensity settings can reduce the ability of the pattern recognition system to locate the references accurately. It is very much desired to have a consistent level of brightness and image quality across all the machines used.
Problems in wire bonding techniques arise in part from the technology trends to increase the number of leads in a given package and to make IC chip packages smaller. As consequences, the bonding pads located on the chip receive smaller areas and are spaced closer together, and the inner leads of leadframes are made narrower and closer together. These trends demand tighter control of wire bond ball and stitch geometries and placements. For instance, even small bond placement errors may result in device loss.
For the bond machines, errors in x-y tables and motors need to be reduced. At the microscopic level, each axis of each table behaves differently with its own local variations within their usable regime. For instance, an axis might have a 0.5% error in its first half of working distance and a 0.8% error in its second half. Further, a x-y pair might have a global positioning error because of an error in the orthogonality between them; or the tables may exhibit a range of hysteresis errors. These variations become even more threatening as common device programs are shared due to quality enforcement and economic reasons.
The emerging technical problems for automated bond machines can be summarized as follows:
Accuracy: Small ball/fine pitch bonding requires a very accurate system to be able to place the ball completely on the bond pad. The current systems have difficulties in achieving this.
Large variations in illumination settings can lead to variations of the images as seen by the optics and the camera. These variations may affect the ability of the pattern recognition system in locating the homes and bond locations accurately.
The current systems cannot handle x-y table inconsistencies. For small pad/fine pitch bonding, a small error in ball placement can cause the ball to be partially off the pad.
Human error during regeneration of alignment program: Ball placement is greatly affected by the accuracy of the alignment program. There are many steps to this regeneration process and thus there are many chances for human error.
Time spent performing alignment regeneration: Whenever a device is to be bonded, a human operator typically has to spend a finite amount of time to perform an alignment regeneration.
An urgent need has therefore arisen for a fast, reliable and flexible system and method to reduce set-up time, reduce errors during creation and retrieval of bonding programs, compensate for machine variability, and standardize illumination conditions on bonding machines. The system and method should be flexible enough to be applied for different IC product families with a wide spectrum of design variations, and for different bond machines. The system and method should spearhead solutions toward the goals of improved product yield and reliability, preferably without investment in new equipment.
The present invention provides a computerized system and method for reducing bond program errors in a slave bonder, prepared to attach connecting bonds onto bond pads of a slave integrated circuit, by first collecting, on a master bonder, input data concerning bond x-y locations, alignment reference x-y locations, and alignment reference images from a master integrated circuit, then analyzing these data to construct a network of relationships between reference images and bond locations, and store data and relationships in a master file. Secondly, on a slave bonder, all this information is automatically retrieved and compared by a computer with input data concerning alignment reference images from a slave circuit. Thirdly, any discrepancy found is corrected by a computer to identify the new bond locations on the slave circuit. Finally, the slave bonder attaches the connecting bonds based on the computed correct bond locations.
The present invention is related to high density ICs, especially those having high numbers of input/outputs and tight constraints in package outline and profile. These ICs can be found in many semiconductor device families such as processors, standard linear and logic products, digital and analog devices, high frequency and high power devices, and both large and small area chip categories. Since the invention aims at designing devices with minimum geometries and high reliability, it supports continually shrinking applications such as cellular communications, pagers, hard disk drives, laptop computers and medical instrumentation.
It is an object of the present invention to provide an automated system and method for retrieving master information and comparing it with the actual bond pad locations on the circuit-to-be-bonded, followed by automated corrections, while entering the master input data manually by an expert. Constructing the network of relationships between the entered data is also computerized. The object is achieved by an embodiment of the invention using a computer system and a computer-implemented method for automatically collecting, analyzing and storing the needed information.
Another object of the present invention is to provide a highly flexible system and method. This object is achieved by the embodiments of four subsystems of the invention:
A user-friendly manual input data generator operable to select alignment reference points, reference images, and bond pad points from a master circuit belonging to the same device family as the circuit-to-be-bonded (xe2x80x9cslave circuitxe2x80x9d).
A computerized analysis generator operable to establish geometric relationships, expressed in x-y and polar coordinates, between the master circuit bond pads and reference structures, and to store all data and relationships in a master file.
A computerized retriever operable to download these data and relationships to a slave bonder designated to perform the bonding processes on the slave circuit.
A comparative corrector operable to compare alignment reference images from the master file to reference image inputs generated from the slave circuit, and then to correct any shift, rotations or scalings between these two images or image parts. After the computational bond pad corrections, the slave bonder is ready to perform the bonding processes on the re-computed pads of the slave circuit.
Another object of the present invention is to provide the newly computed bond locations in fast turn-around time and with minimum effort by taking full advantage of symmetries and branching in the computational flows of the input and analysis generators, the retriever, and the corrector.
Another object of the present invention is to introduce bond program teaching, loading (xe2x80x9cregenerationxe2x80x9d) and correcting concepts which are flexible so that they can be applied to many families of electronic structuresxe2x80x94reaching from piece parts, such as leadframes and interconnectors, to device packages, to electronic substrates, and to whole assemblies on motherboardsxe2x80x94and are general so that they can be applied to several generations of products. Beyond the electronics realm, the computerized system and method of this invention can be generally applied to reduce operational program errors on slave machines prepared to work on action sites of slave objects, when a master machine and a master object, having a structure similar to the slave object, are available.
The technical advances represented by the invention, as well as the objects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.